look up any word, like wyd:
Laughing Silently Internally . This acronym is meant to take the place of LOL which stands for Laughing Out Loud. It is my belief, and I think I can speak for millions of others, that the acronym LOL is overused and not meaningful. People have overused it as a quick and easy way to say that they find something funny. They are probably not really laughing out loud. LSI is a way to quickly say that you find something funny but being truthful. Now when you say LOL it will have even more meaning.
example: a facebook post
This video is so funny. LSI.

example a response to text message
You are retarded! LSI

As an adjective:
I was checking out this blog and it was funny. Had me LSIing

now you can use LOL when more appropriate:
When you told me you were destined for greatness you had me LOL. I was drinking a cola at the time and shot right out of my nose!
by The Stroz January 15, 2014
Life Saving Instructor. A Person who is able to instruct lifesaving courses
With my LSI I was able to teach Bronze cross.
by GSS_NS November 08, 2006